Ion exchange membrane is in wide use as a membrane for cell (e.g. polymer electrolyte fuel cell, redox flow cell or zinc-bromine cell), a membrane for dialysis, etc. Polymer electrolyte fuel cell using an ion exchange membrane as the electrolyte is an power generation system in which a fuel and an oxidizing agent are fed continuously, then they are reacted, and the resulting chemical energy is taken out as an electric power; and it is an power generation system which is clean and highly efficient. In recent years, this power generation system has increased its importance for uses in automobile, household and portable device because it can be operated at low temperatures and can be produced in a small size.
Polymer electrolyte fuel cell has, in general, a solid polymer membrane functioning as an electrolyte. Onto the both sides of the solid polymer membrane are bonded a gas diffusion electrode having a catalyst loaded thereon. In this fuel cell, a fuel (which is hydrogen gas, methanol or the like) is fed into a chamber (fuel chamber) in which one of the gas diffusion electrodes is present, and an oxygen-containing gas as an oxidizing agent (e.g. oxygen or air) is fed into a chamber in which the other gas diffusion electrode is present. When, in this state, an external load circuit is connected to the two gas diffusion electrodes, the fuel cell works as such.
Of fuel cells, direct methanol fuel cell in which methanol is used per se as fuel, is easy to handle because the fuel is a liquid, is inexpensive, and, therefore, is expected as a electric power source of relatively small output, used especially for portable device.
The fundamental structure of polymer electrolyte fuel cell is shown in FIG. 1. In FIGS. 1, 1a and 1b are partition walls of fuel cell, provided so as to face each other. 2 is a groove-shaped fuel passage formed in the inner surface of the partition wall 1a. 3 is a groove-shaped oxidizer gas passage formed in the inner surface of the partition wall 1b. 6 is a solid polymer electrolyte membrane; on one side thereof is formed a fuel chamber side diffusion electrode layer 4 and on the other side is formed an oxidizer chamber side gas diffusion electrode layer 5. The solid polymer electrolyte membrane 6 electrically insulates a fuel chamber 7 from an oxidizer chamber 8; however, proton permeates the solid polymer electrolyte membrane 6.
The principle of this polymer electrolyte fuel cell is explained on a case of proton-conductive type fuel cell which uses a cation exchange membrane as the solid polymer electrolyte 6. The hydrogen or methanol fed into the fuel chamber 7 reacts at the fuel chamber side diffusion electrode layer 4, generating proton (hydrogen ion) and electron. The proton passes through the inside of solid polymer electrolyte membrane 6 and reaches the oxidizer chamber 8, where the proton reacts with the oxygen in air or oxygen gas, generating water.
Meanwhile, the electron generated at the fuel chamber side diffusion electrode layer 4 passes through an external load circuit (not shown) and reaches the oxidizer chamber side gas diffusion electrode layer 5. At this time, the external circuit is provided with an electric energy.
Ordinarily, in a polymer electrolyte fuel cell having such a structure and in a case of proton-conductive type fuel cell, a cation exchange membrane is used as the solid polymer electrolyte membrane. In a case of anion-conductive type fuel cell, an anion exchange membrane is used as the solid polymer electrolyte membrane. These ion exchange membranes are required to have low electrical resistance, high water retention, stability during long-term use and high physical strength.
As such an ion exchange membrane, there has been mainly used a non-crosslinked perfluorocarbonsulfonic acid membrane when the ion exchange membrane is, for example, a cation exchange membrane. This membrane has high chemical stability. However, being insufficient in water retention, the membrane tends to dry and resultantly show reduced proton conductivity. Further, since the membrane is insufficient in physical strength, it is difficult to allow the membrane to have a small thickness for lower electrical resistance. Furthermore, when the membrane is used as a membrane for direct methanol fuel cell, there tends to occur a problem of phenomenon that methanol permeates the membrane, i.e. a so-called methanol cross-over phenomenon.
In order to solve these problems, researches on ion exchange membranes other than the perfluorocarbonsulfonic acid have been under way actively, in recent years. As one of such ion exchange membranes, there is a so-called hydrocarbon-based solid polymer electrolyte membrane. As an example, it was proposed to use an ion exchange membrane obtained by using, as a substrate, a porous film made of polyethylene or the like and integrating this substrate with an ion exchange resin. As the ion exchange resin, hydrocarbon-based ion exchange resins such as polystyrenesulfonic acid and the like are in use. Ordinarily, these hydrocarbon-based ion exchange resins have a crosslinked structure formed by copolymerizing bi- or higher functional crosslinking monomers such as divinylbenzene and the like. An ion exchange membrane obtained by integration of a hydrocarbon-based ion exchange resin having such a crosslinked structure and a substrate, is good at dimensional stability, heat resistance and mechanical strength. Further, the ion exchange membrane containing a substrate is greatly suppressed in methanol permeability (reference is made to, for example, Patent Literatures 1 and 2).
In order to further increase the proton conductivity of ion exchange membrane and further reduce the methanol permeability of ion exchange membrane, there is also known an ion exchange membrane impregnated with a polymer having a charge group having a polarity opposite to that of the ion exchange group possessed in the ion exchange membrane. In this ion exchange membrane, the ion exchange group and the charge group of polarity opposite to that of the ion exchange group form an ionic bond inside the membrane. In this ion exchange membrane, there is used, as the polymer having a charge group of opposite polarity, a liquid polymer or a polymer of relatively low molecular weight dissolved in an organic solvent. This technique is employed also in the hydrocarbon-based ion exchange membrane and there is disclosed a hydrocarbon-based ion exchange membrane having an ion pair formed therein (reference is made to Patent Literatures 3 and 4).
In producing a fuel cell using, as the solid polymer electrolyte membrane, a crosslinked hydrocarbon-based, ion exchange membrane which, as mentioned previously, is low in methanol permeability and superior in dimensional stability, heat resistance, etc., there is a problem that bonding is insufficient between the electrolyte membrane and the fuel chamber side gas diffusion electrode layer and oxidizer chamber side gas diffusion electrode layer, bonded to the both sides of the electrolyte membrane.
Each of the above catalyst electrode layers is ordinarily formed with a catalyst such as platinum or the like, an electron-conductive substance such as conductive carbon or the like, and an ion-conductive substance such as cation exchange resin, anion exchange resin or the like. In bonding the catalyst electrode layer, ordinarily, the above materials are kneaded using a dilution solvent to produce a paste, at first. Then, the paste is coated on the surface of the solid polymer electrolyte membrane, followed by drying and hot-pressing, whereby catalyst electrode layers are bonded to the electrolyte membrane.
When the solid polymer electrolyte membrane is a non-crosslinked ion exchange membrane such as the above-mentioned perfluorocarbonsulfonic acid membrane or the like, the hot-pressing can strongly fusion-bond the catalyst electrode layer to the solid polymer electrolyte membrane. However, when the solid polymer electrolyte membrane is, for example, a cross-linked hydrocarbon-based ion exchange membrane, no sufficient fusion-bonding takes place and the bonding strength between the electrode layer and the electrolyte membrane is significantly low.
When the bonding between the solid polymer electrolyte membrane and the catalyst electrode layer is insufficient, the ion conductivity at the bonding interface between them is low. When a fuel cell is produced using a membrane for fuel cell, of insufficient bonding, the internal resistance of the fuel cell is large. Also, even when the bonding interface between the membrane and the catalyst electrode layer has a relatively good ion conductivity at the initial stage of fuel cell use, the bonding between the membrane and the catalyst electrode layer is reduced further with the passage of use period owing to, for example, the swelling of bonded portion by methanol. As a result, there occurs a problem that the catalyst electrode layer portion is peeled from the solid polymer electrode membrane in a relatively short period.
This problem is improved slightly when the ion exchange membrane is subjected to a treatment of impregnation with a polymer having a charge group of polarity opposite to that of the ion exchange group of the membrane. In these known techniques, the above-mentioned polymer is impregnated into the solid polymer electrolyte membrane, whereby an ion pair is formed near the surface of the electrolyte membrane and a composite layer is formed (Patent Literature 3). The polymer used is a liquid polymer having a molecular weight of several hundreds, which is impregnated easily into the electrolyte membrane (Patent Literature 4). The most part of the polymer impregnated is present inside the electrolyte membrane and the amount of the polymer having a charge group of opposite polarity, present on the surface of the electrolyte membrane is very small. Therefore, the above method for obtaining higher bonding strength by polymer impregnation for formation of ion pair using a charge group of opposite polarity and resultant strong bonding between the electrolyte membrane and the catalyst electrode layer is not effective.
As described above, there are still various inconveniences in order to use the above-mentioned, cross-linked hydrocarbon-based ion exchange resin as a membrane for fuel cell; and there remain tasks to be overcome, such as strong bonding with catalyst electrode layer, superior ion conductivity of bonding portion, low methanol permeability, dimensional stability, heat resistance and the like.
Patent Literature 1: JP-A-1999-335473
Patent Literature 2: JP-A-2001-135328
Patent Literature 3: JP-A-2001-167775
Patent Literature 4: JP-A-2001-236973